1. Field of the Invention
The present invention relates to a dielectric ceramic material for use in a high-frequency wave region and a method of producing the same, and more particularly, to a dielectric ceramic material having a high Q value and good temperature stability, which can be effectively used as a dielectric resonator, and a method of producing same.
The dielectric ceramic materials are widely used as dielectric resonators in microwave and millimeter wave, frequency. The dielectric resonators are elements of micro, millimeter wave components such as dielectric filters and dielectric resonator stabilized oscillators. As examples of their recent application, there are antenna band pass filters, narrow band filters, and dielectric local oscillators of SHF-TV converters.
To replace metal cavity resonators by ceramic dielectric resonators, it is possible to reduce their dimensions which results in light weight and low cost, because the permittivity of such ceramics is higher than that of air. Especially, the application of dielectric ceramic materials with high Q values to the dielectric resonators is of great advantage to ensure high frequency stability and good noise characteristics of the microwave components.
The band pass and band stop filters and the local oscillators are used as transmitting and receiving components for the electric wave in broadcasting or communication satellites system, and further, in radar and telecommunication systems. As an example of another application, since the dielectric ceramic material has a relative permittivity which is twice or more than that of alumina and has a relative frequency stability of a few ppm/.degree.C., the material can be applied to a substrate material for microwave integrated circuits.
2. Description of the Related Art
A dielectric ceramic material used as a dielectric resonator in a high frequency wave region must have small dielectric losses whose magnitude is determined by the loss factor tan .delta., where .delta. is the loss angle of the material. In resonators the dielectric loss is expressed in the Q-factor, which is defined as the reciprocal of the loss factor. Especially, when the dielectric resonator is not terminated by the load, the Q-factor is called an unloaded Q. The magnitude of the unloaded Q is strongly dependent on properties of the material, and must be kept as large as possible. The large Q value of materials enables a local oscillator to stabilize a frequency load change or a frequency voltage change. Further, use of a high Q material enables the design of a high-power oscillator and narrower band pass or stop filters. The temperature coefficients for the resonant frequency of dielectric resonators must be within a few ppm/.degree.C., in order to stabilize the band frequency of filters and the constant frequency of oscillators for service conditions.
Since the cavity resonator at the same frequency
and height/diameter ratio is about .sqroot..epsilon..sub.r times as large as the dielectric resonator, the relative permittivity .epsilon..sub.r of the material must be high, to keep the dimensions of the resonator within resonable limits.
As dielectric ceramic material, Ba(Zr, Zn, Ta)O.sub.3, BaO-TiO.sub.2, ZrO.sub.2 -SnO.sub.2 -TiO.sub.2 or Ba(Zn, Ta)O.sub.3 -Ba(Zn, Nb)O.sub.3, etc., are well known. Such dielectric ceramic materials have relative permittivities of about 20 to 90, and Q values of 3000 to 10000, and temperature coefficients of .+-.10 ppm/.degree.C. or less at a frequency of 10 GHz.
Due to recent developments in communication techniques, however, a material with a higher Q value is desired in dielectric resonators, in order to realize the narrower frequency band in band pass/stop filters or the use of higher frequency band more than 10 GHz. The latter requirement is based on the property expressed by f.multidot.Q=const, that is, even in a similar device, with the increase of resonant frequency, the Q.multidot.factor is degraded in dielectric resonators.
As a material having a high Q value, a Ba(Mg.sub.1/3 Ta.sub.170 )O.sub.3 material has been disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-170102. It is reported that this material has a high value, i.e., 34000, at 10 GHz and a sintering density of 95% obtained by a two step heating treatment, which consists of rapid sintering with a high heating rate (the first step of the heat treatment) and next annealing in an oxygen atmosphere (the second step).
This production process of Ba(Mg.sub.1/3 Ta.sub.2/3)O.sub.3, compound is, however, based on the rapid sintering method, in which the heating rate is about 1600.degree. C./min. Furthermore, an oxygen atmosphere is required during the second step of the heat treatment. From the view points of responsibility and production costs many problems will occur in actual manufacturing.
On the other hand, as is well known, a Ba(Sn, Mg, Ta)O.sub.3 dielectric ceramic material is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-124305, a Ba(Zn, Mg)(Nb, Ta)O.sub.3 dielectric ceramic material is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 60-68503, and a (Ba, Sr)(Mg, Ta)O.sub.3 dielectric ceramic material is disclosed, in Japanese Unexamined Patent Publication (Kokai) No. 63-37508, in which the sintering has been done in air with a normal heating rate such as 200.degree. C./min. Such dielectric ceramic materials, in which various additives are used, have been improved their sintering properties, but the Q values thereof are about 10000, which is not high enough.